Saturating doses of crystalline insulin, or a physiological ration of 4 mm sodium D-B hydroxybutyrate and 1 mm acetoacetate, or both were administered to rat hearts perfused with modified Krebs-Henseleit buffer containing 10 mm glucose. During 30 minutes of perfusion, hydraulic work, 02 consumption, L-lactate output and the rate of 3H20 production from [2- 3H] D-glucose were measured. At the end of 30 minutes, the hearts were freeze-clamped and the intracellular contents of the glycolytic intermediates and their associated nucleotides were measured. In another set of animals, the kinetic parameters of 13 enzymes of glycolytic and glycogen metabolism were measured. Cardiac efficiency was increased from 10 to 14% by the addition of ketones, insulin, or the combination. 02 consumption was decreased by insulin or insulin plus ketone from 5.6q0.1 and 5.4q0.4 umol/min/g wet wgt, respectively. Net glycogen breakdown was 0.15 umol glucose units/min/g wet wgt. in the presence of glucose alone; in the presence of ketones or insulin there was a net synthesis equivalent to 0.22 and 0.80 umol/min/g, respectively. Flux through the P- glucoisomerase was decreased 2-fold by the addition of ketones, 3-fold by insulin and 2-fold by the combination. Kinetic parameters of the glycolytic enzymes indicate that most of the enzymes are not present in the amounts that greatly exceed influx. Major disequilibrium in the glycolytic pathway shifts from glucose transport when hearts are perfused with glucose alone, to the P-glycerate mutase reaction after addition of ketones, insulin or both.